1. Field of the Invention
The invention relates to a bonding assistant for a support material, consisting of an organic binder.
2. Brief Description of the Prior Art
The problem of adherence arises especially in the manufacture of conductor paths in printed circuits. In practice a plurality of methods of manufacturing these printed circuits already exist. Apart from the subtractive method, also the additive or the semiadditive method, respectively, are widely used. In all methods of additive build-up which have already become known, a bonding assistant is needed which is to insure adherence between the support material or substrate and the additively built-up metal layer, being in most cases a copper layer, even if these methods are distinguished by different process steps. This requirement applies especially when the conductors paths, as usual, are to be solderable. The bonding assistant itself is a thermosetting adhesive, generally on the basis of an acrylonitrile/butadiene copolymer, i.e. a nitrile rubber which is cured with a heat-reactive phenolic compound. To obtain sufficient bond strength with respect to the copper coating on the substrate, the bonding assistant often is subjected to a chemical etching treatment. Thereafter, e.g., the support material is activated in the usual manner and then chemically copper-plated. Thereafter a negative image is formed in a manner known per se by screen or photo-printing. By subsequent galvanic copper-plating the desired conductor path thickness can be obtained. To obtain a still better bond strength, it is often required to subject the support material coated with the bonding assistant also to a mechanical roughening treatment.
For example, a method for producing a printed circuit is known (German OS No. 16 40 635), wherein the substrate is provided with a layer of bonding assistant. This layer consists of a mixture of a curable resin such as epoxide or phenolic resin in combination with fine-grained aggregate such as silicates or metal oxides. The layer of bonding assistant is cured at least partially or completely, whereby adherence to the surface of the substrate is insured. Prior to the application of the conductive metal layer, the layer of bonding assistant is subjected to a chemical digestion. Prior to digestion the layer of bonding assistant can be subjected to a mechanical roughening treatment. By this treatment the aggregate particles are partially ground, and during subsequent digestion they are removed by dissolution without the resin portion being affected. Thus a surface provided with holes and undercuts is formed. Onto this surface the conductive metal layer is deposited galvanically. Chemical digestion is effected by means of sodium hydroxide, weak acids or organic solvents. This process step involves a loss of etchant necessary for the digestion process, quite apart from the sewage problems. In addition, precise guidance of the bath with continuous process control and frequent bath renewals are required.
A further method for producing a printed circuit, using the additive technique, is known (German AS No. 20 44 484), in which a bonding assistant is interposed between copper layer and substrate. This bonding assistant is composed of acrylonitrile butadiene and phenolic resin, dissolved in methylethyl ketone. After drying for two hours and a half at 140.degree. C. the bonding assistant exhibits a surface resistance of 10.sup.5 to 10.sup.8 megohms. Prior to copper-plating, the bonding assistant is partially etched by means of a mixture of sulfuric acid, water and chromic acid. By this activation the butadiene component is oxidatively affected more rapidly than the two other components. Thus a molecular hole is formed at the points where a butadiene chain is present, in which thereafter the chemically deposited copper can be anchored.